The present claimed invention relates to the field of digital communication. Specifically, the present claimed invention relates to an apparatus and a method for providing time tracking of a signal using hardware and software.
Wireless telephony, e.g. cellular phone use, is a widely-used mode of communication today. Variable rate communication systems, such as Code Division Multiple Access (CDMA) spread spectrum systems, are among the most commonly deployed wireless technology. Because of increasing demand and limited resources, a need arises to improve their capacity, fidelity, and performance.
Referring to prior art FIG. 1A, a conventional base station 104 and a mobile unit 102, e.g. a cell phone, are shown. A CDMA system uses a common bandwidth to transmit the pilot signal and a data signal 106 between a base station 104 and a mobile unit 102, for multiple users. Hence, the bandwidth is occupied by an amalgam of many signals. In order to extract an individual signal from the amalgam of signals, the individual signal is modulated at the transmitter and demodulated at the receiver.
Referring now to prior art FIG. 1B, two signals having an exemplary PN sequence 100b are shown. The modulation code used for CDMA is a pseudonoise (PN), e.g. an apparently random sequence of 1""s 120 and 0""s 118 having a given length, e.g. 110a. A xe2x80x9clongxe2x80x9d PN sequence is used to scramble user data, while a xe2x80x9cshortxe2x80x9d PN sequence is used to spread quadrature components for the forward and reverse link wave forms.
The short PN sequence is used for a pilot signal. A base station broadcasts a pilot signal to which a mobile receiver may receive and synchronize, for time tracking purposes. Thus, as shown in prior art FIG. 1B, a first signal 114a of a conventional PN sequence, e.g. 110a, is repeatedly generated, e.g. 110b, by a first device such as a mobile unit 102 of prior art FIG. 1A. Similarly, the same conventional PN sequence, e.g. 112a, is repeatedly generated, e.g. 112b, by a second device such as a base station 104 of prior art FIG. 1A.
The mobile unit and base station can communicate to each other when they properly align the starting points of the known PN sequence. Proper alignment is utterly critical, in-applications such as CDMA, because it is necessary for accurately applying the PN sequence to demodulate a signal out of the amalgam of signals that occupy the bandwidth. While a signal may be correctly aligned at the beginning of the signal, e.g. 116, it may not be properly aligned throughout the entire signal, e.g. offset error 118, as shown in prior art FIG. 1A. The misalignment can arise from a difference in the frequency Vcxo of the PN sequence generated at each unit. This is referred to as a frequency offset between a base station and a mobile unit. Hence a need arises for time tracking a signal generated by one device with a signal received from another device to ensure proper alignment of the signals for modulation and demodulation.
Determining the alignment of the signals can be accomplished by performing digital signal processing (DSP), such as a correlation operation, on the signals. However, imperfections in transmitting, receiving, modulating, and demodulating a signal, inherently integrate noise into the signal. The noise hampers, and sometimes prevents, the DSP operation to demodulate a signal and receive the data contained therein. To improve the time tracking operation, a need arises to remove the noise from the correlation data used for time tracking.
Referring now to prior art FIG. 1C, a conventional time tracking block diagram is shown. Conventional time tracking uses hardware 254 to perform the time tracking operations and provide input to system timing 256. Conventionally, the time tracking hardware removes noise from the signal by passing the signal through a hardware filter 254a. However, by using hardware, the conventional filter 254a has a fixed configuration. That is, the conventional filter has a single configuration that cannot be changed, unless the hardware is redesigned and replaced. But different operating circumstances and resources for a communication device or a communication system may allow or demand higher or lower performance filters. To optimize performance and resources, a need arises to provide a flexible filter for removing noise from data used for time tracking.
Once a signal is correlated and normalized, it can be compared to a threshold. Conventional methods divide correlation data by a normalizing value. However, a division operation is more difficult to implement in an electronic component. Consequently, a need arises to simplify the normalization operation of a threshold value used for time tracking.
Conventionally, a filter used for correlation data is not reset after a timing correction is made. Consequently, the filter retains and evaluates some obsolete data for a subsequent timing correction. This practice can provide incorrect timing corrections and possibly lead to timing oscillation. Overall, the conventional practice degrades system performance. Hence, a need arises for a method to avoid filtering obsolete data once a timing correction has been made.
In summary, an apparatus and a method is needed to improve the capacity, fidelity, and performance of digital communication. More specifically, a need arises for time tracking a signal generated within a mobile device to a signal received from a base station to ensure proper alignment of the signals. To improve the time tracking, a need arises to remove the noise from the correlation data used for time tracking. To optimize performance and resources, a need arises to provide a flexible filter for removing noise from pilot channel used for time tracking. Also, a need arises for a method to avoid filtering obsolete data once a timing correction has been made. Additionally, a need arises to simplify the normalization operation of a threshold value used for time tracking.
The present invention provides a method and apparatus for improving the capacity, fidelity, and performance of digital communication. More specifically, the present invention provides improved time tracking between a signal from a base station and a signal generated within a mobile device. The present invention improves time tracking by remove noise from the correlation data used for time tracking. To optimize performance and resources, the present invention provides a flexible filter for removing noise from correlation data used in time tracking. Additionally, the present invention provides a method to avoid filtering obsolete data once a timing correction has been made, thereby improving system performance. Additionally, the present invention provides a more efficient method of normalizing a threshold value used for time tracking.
In one embodiment, the present invention recites a method comprising several steps. In one step, a communication device, such as a cell phone, generates a first signal and receives a second signal from an outside source, such as a base station. Both signals are designed to have the same finite and repeating, but not necessarily aligned, pseudonoise (PN) sequence that can be used to synchronize the timing between the two devices. Next, correlation data between a first signal, at a plurality of timing conditions, and a second signal is generated by hardware in the communication device. The plurality of timing conditions allows an on-time condition, an early condition, and a late condition of the first signal to be evaluated for alignment with the second signal. By evaluating several timing conditions, the direction for optimal correlation between the signals can be determined. The correlation data is filtered by software or firmware at the plurality of timing-conditions to remove noise and thereby provide more accurate correlation data for subsequent evaluation. By implementing the filtering function using software or firmware, the present invention provides a very flexible and adjustable filter. For example, the filter coefficients can be quickly and inexpensively modified without hardware modifications. Thus the present invention overcomes the drawbacks of the prior art filter which was implemented in hardware.
After the filtering step, the correlation data is normalized and compared to a threshold value to obtain a result that represents the accuracy of a system timing for the first signal. One embodiment normalizes the threshold value using a more efficient multiplication operation in lieu of the conventional division operation. Specifically, the threshold value is multiplied by the on-time energy value resulting from correlation between the first signal and the second signal. This normalized threshold value is compared to the difference between the early energy and the late energy calculation resulting from correlation between the first signal to the second signal. Finally, the system timing for the first signal is corrected based upon the result of the comparing step.
In another embodiment, the present invention recites a communication device including hardware, a processor and a computer readable memory. The memory contains program instructions that, when executed via the processor, implement the aforementioned method for determining translation error in a stepper.